1. Field of the Invention
The present invention relates to a slewing type construction machine such as a hydraulic excavator.
2. Description of the Related Art
Background art for the present invention will be described taking a hydraulic excavator shown in FIG. 3 as an example. The hydraulic excavator includes a crawler type lower traveling body 1, an upper slewing body 2 installed on the lower traveling body so as to be able to slew around an axis X perpendicular to a ground surface, and a front attachment 6 for excavation, the front attachment 6 attached to the upper slewing body 2. The front attachment 6 has a boom 3, an arm 4, and a bucket 5. Moreover, the hydraulic excavator includes a hydraulic slewing system for hydraulically slewing the upper slewing body 2. The system includes a slewing motor including a hydraulic motor and serving as a drive source, a control valve that controls operation of the slewing motor, right and left slewing conduit lines that connect the hydraulic motor to the control valve, a relief valve provided between the slewing conduit lines to serve as a brake valve, a remote control valve including an operation lever to which operations for slewing are applied, and a slewing brake that is a mechanical brake, i.e., what is called a parking brake. In the system, upon return of the operation lever to a neutral position, a supply of oil from a hydraulic pump to the slewing motor is stopped. Furthermore, the relief valve is opened, that is, relief actuation is performed to decelerate the slewing motor, and the slewing brake is actuated to keep the upper slewing body 2 stopped.
Moreover, various control operations are performed based on the hydraulic slewing system. For example, the slewing speed of the upper slewing body is detected by use of a slewing speed sensor. Based on the detected slewing speed, the actuation of the hydraulic slewing system is controlled. For example, Japanese Patent Application Laid-Open No. 2011-179280 discloses a technique of shifting to a constant speed operation when the detected slewing speed reaches a target speed determined from the operation amount of the remote control valve. Also a technique for controlling the discharge rate of the hydraulic pump in accordance with the slewing speed is known.
However, in connection with the characteristics of the slewing speed sensor, an offset (deviation from specifications) may occur in sensor output as a result of a change in temperature or the like. Specifically, a phenomenon may occur in which the sensor output fails to become zero even though the upper slewing body is stopped. Such a phenomenon may disadvantageously inhibit the slewing speed from being accurately detected, involving a problem of failing to intended control from being achieved or reducing control accuracy due to the use of erroneous sensor output.